Here above is the explanation printed on the back of the chart. For example: At what time the tide will be out September the 9th? (which by the way is the beginning of the St.Ives September Festival).
Easy. September... is purple. Check out where the number 9 is placed on the purple circle...It's just before the number 14 and 15 of the 24 hour clock. That's 2: 30pm.
Another example: The festival goes on until September 23rd.
Purple ring...23...ah, it's at 2.30 in the morning. That means that the high tide is at about 8.30am...